1. Field of the Invention
The present invention relates to ultrasonic diagnostic apparatus in which ultrasonic pulses are radiated toward and reflected from objects within a living body and more particularly is directed toward compensating for attenuation of such ultrasonic pulses due to absorption and scattering of the latter by these objects.
2. Description of the Prior Art
For purposes of medical diagnosis, a beam of ultrasonic waves of short duration, that is, ultrasonic pulses produced by an ultrasonic source such as one or more electroacoustic transducers are directed toward and focussed at a fetus, a particular internal organ, or other object within a patient under examination. As the pulse travels through the patient, portions of each pulse are reflected back toward one or more receiving elements, such as the ultrasonic source, as the acoustic impedance along the path of the pulse changes. Variations in the acoustic impedance, which is defined as the product of the density of the object through which the pulse travels and the speed of the ultrasound pulse, occur at tissue interfaces. These reflections are converted by the one or more receiving elements into electrical signals, referred to as echo signals, which are amplified and further processed and then displayed on a screen, such as a cathode ray tube.
The strength, that is, amplitude of the echo signals depends on the length and path traveled by the ultrasonic pulses. More particularly, as the beam travels toward and is reflected from an object of interest, portions of the beam are absorbed and/or scattered by the object of interest and/or other objects along the path of the beam. Consequently, the amplitude of the echo signals is reduced resulting in a distored image produced on the screen.
The strength of the echo signals, however, can be compensated and thereby corrected for by providing a variable gain amplifier whose gain changes with time. For controlling the gain of the amplifier, a control voltage generator such as disclosed in U.S. Pat. Nos. 4,016,862; 4,140,107 and 4,205,555 provides integrating means which produce a control voltage in the form of a ramp supplied to the amplifier in order to vary the gain with time. Initiation of the ramp voltage in U.S. Pat. Nos. 4,016,862 and 4,140,107, is triggered at the moment the beam is radiated from an ultrasonic source. Consequently, the gain of the amplifier is continuously increased beginning at the moment the beam is radiated from the ultrasonic source.
The ulrasonic source in all of these patents is designed so as to rest on the skin surface of the patient. Accordingly, the beam initially travels through the skin surface and does not immediately come into contact with objects which would significantly absorb or scatter the beam. Therefore, increasing the gain of the amplifier at the moment the beam is radiated from the ultrasonic source, as in U.S. Pat. Nos. 4,016,862 and 4,140,107, is unnecessary and undesirable. In fact, applying a ramp voltage to the amplifier before the echo signal has been significantly attenuated overcompensates for such attenuation and provides an echo signal with too high an amplitude, thereby distorting the image produced on the screen.
In U.S. Pat. No. 4,205,555, operation of the integrating means is inhibited by delay circuitry so that a ramp voltage can be supplied to the amplifier only after an acceptable period of time has elapsed from transmission of the beam. The delay circuitry, however, is dependent upon receiving a pulse from a pulse generating means for purposes of triggering operation of the delay circuitry. In other words, U.S. Pat. No. 4,205,555 discloses an undesirable nonautomatic rather than a desirable automatic means of delaying the time varying characteristics of the control voltage supplied to the amplifier.